An estimation of the abundance of plastic litter generated by beach users nationwide in Japan

Coastal regions, including beaches, constitute major tourism assets. Concurrently, beaches are hotspots for microplastic generation, and accumulated beached litter substantially influences future microplastic abundance in the marine environment. Although the stock of plastic litter on beaches has been estimated in previous studies, knowledge gaps exist with regard to the amount of annually generated litter by beach users and the absence of litter generation rate (LGR) in g/person/h or items/person/h. This study aimed to estimate beach-based litter using a novel method. We collected beach-based plastic litter and measured the person-hours of the users using regular photography or videography in summer and autumn/winter at four popular beaches in Japan to obtain LGR and estimate the total litter abundance generated annually from 770 Japanese beaches as a baseline. In summer (autumn/winter), LGR ranged from 0.178 (0.128) to 1.10 (0.453) g/person/h. The LGR in weight was on a 10^-1 order regardless of the season and site, except for one survey (1.1 g/person/h). In number, it ranged from 0.064 (0.042) to 0.196 (0.122) items/person/h. Using mobile phone big data to assess the person-hours of users on 770 beaches, we estimated the total abundance of litter generated by beach users nationwide for the first time owing to these smaller variances in LGRs. The mean weight (number) was estimated as 12.50 t/y (2.80 million items/y) with a 95 % confidence interval of 6.54-18.40 t/y (1.70-3.89 million items/y). These findings would assist in the development of effective strategies for beach debris management.